In recent years, methods have been developed for detecting and determining the quantity of a micro substance existing in a solution, e.g. a bio-related substance, such as DNA, RNA, protein, a virus or a bacterium, by conjugating a fluorescent material, as a label, to such a bio-related substance and by exciting the emission of light from the label by using an enhanced electric field on the surface of a sensor chip. Further, there has been developed a method for fluorescently staining a tissue in a cell to fluorescently observe the stained particular region by using an enhanced electric field on the surface of a sensor chip. As a technique for generating the enhanced electric field used in the foregoing methods, a surface plasmon resonance (SPR) is well known.
The SPR uses an optical configuration referred to as Kretschmann configuration, in which the SPR is excited on a gold thin film on the surface of glass in contact with a prism by the total reflection of incident light at the interface between the gold thin film and a liquid sample thereby to form an enhanced electric field on the surface of the gold thin film. SPR-excitation-enhanced fluorescence spectroscopy is a technique for performing fluorescence observation with less background light by using light that has been enhanced in the vicinity of the surface of a gold thin film by the SPR as the excitation light to excite fluorescent molecules existing in an enhanced electric field thereby to generate intense fluorescence (refer to Patent Document 1).
As with the SPR, an excitation mechanism of a guided mode (also referred to as an optical guided mode, a waveguide mode, an optical waveguide mode or the like) is well known as a mechanism capable of forming an enhanced electric field on the surface of a sensor chip (refer to Patent Documents 2 to 5 and Non-Patent Documents 1 to 6).
A configuration example of a conventional guided mode excitation mechanism is illustrated in FIG. 13. FIG. 13 illustrates a guided mode excitation mechanism using the Kretschmann configuration. The mechanism uses a detection plate 4 composed of a light transmissive substrate 9 (a sheet glass or the like), a metal layer or a semiconductor layer 10 formed thereon, and a light transmissive dielectric layer 11 further formed thereon. An optical prism is closely attached to the surface of the detection plate 4 on the opposite side from the surface on which the light transmissive dielectric layer 11 is formed (the interface relative to a sample 5) through the intermediary of a refractive index adjustment oil or an optical adhesive agent, and white light or laser light is irradiated from a first light irradiation unit 1 onto the detection plate 4 through a prism 3. The incident light is incident on the detection plate under a total reflection condition. At a particular incident angle, incident light having a particular wavelength is conjugated with a guided mode propagated in the layer structure on the detection plate, thus exciting the guided mode. When the guided mode is excited, the electric field of the light having the particular wavelength is enhanced on the front surface of the detection plate, thus forming an enhanced electric field. The wavelength of the light at which the enhanced electric field has been formed can be verified by measuring, by using a spectroscope, the wavelength at which the intensity of the reflected light has been attenuated.
In the detection plate, the guided mode excited in the detection plate having a metal layer formed on a light transmissive substrate is frequently referred to as a leaky mode, leakage mode or the like (refer to Non-Patent Document 7). Further, in a detection plate, it has been known that using silicon or germanium or a mixed material thereof is effective for the semiconductor layer formed on the light transmissive substrate. The light transmissive dielectric layer may be composed of a single dielectric material. However, it has been reported that an enhanced electric field of a higher electric field intensity can be formed if the light transmissive dielectric layer is formed by stacking a plurality of dielectric materials in laminate (refer to Non-Patent Documents 8 and 9).
While only p-polarized light can be used for the excitation of the SPR, both p-polarized light and s-polarized light can be used for the excitation of the guided mode. The wavelength of light at which the enhanced electric field is obtained by the excitation of the guided mode is determined by the refractive index of the substrate of a detection plate, the complex refractive index and the thickness of each layer, and the incident angle of incident light and the direction of polarization of incident light. Depending on these conditions, the guided mode is excited in a plurality of wavelength bands at one time. For example, if 0.75 mm-thick silica glass is used as a substrate, a 200 nm-thick silicon layer is used as a semiconductor layer, a 300 nm-thick SiO2 glass layer is used as a light transmissive dielectric layer, and s-polarized white light is irradiated at a 67-degree incident angle onto the front surface of the detection plate through a silica glass prism, then three guided modes having electric field intensity peaks at wavelengths of 513 nm, 588 nm, and 736 nm are excited in a visible light region, as illustrated in FIG. 14.
Further, there has been reported a device in which a substrate is provided with a groove, and detection areas having inclined surfaces with different angles are formed on the bottom surface or a side surface of the groove, and different substances are optically detected on the individual inclined surfaces (refer to Patent Documents 6 and 7).